A PAS typified by polyphenylene sulfide (hereinafter abbreviated as “PPS”) is an engineering plastic excellent in heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical properties, dimensional stability and the like. The PAS can be molded to various moldings, films, sheets, fibers and the like by typical melt processing methods such as extrusion molding, injection molding, compression molding and the like. In addition, the PAS can be used for coatings on other materials such as metal. Therefore, the PAS is used in wide application fields such as electric and electronic equipment and automobile equipment.
A PAS is generally produced by a method in which a dihalo-aromatic compound and an alkali metal sulfide are polymerized (polycondensed) by heating in an organic amide solvent. A polymerization aid, a phase separation agent or the like is added as required before the polymerization process, during the polymerization process or after the polymerization process.
Various additional processes are provided before and after the polymerization process of PAS. The polycondensation reaction of a dihalo-aromatic compound and an alkali metal sulfide is susceptible to water, but the alkali metal sulfide as a raw material contains much amount of water such as the water of crystallization in many cases. Therefore, before the polymerization process of PAS, there is generally provided a dehydration process in which the water in the reaction system is adjusted by distilling water out of a mixture containing an organic amide solvent and an alkali metal sulfide. After the polymerization process, there is provided a process in which the polymerization reaction system is cooled and the PAS produced is recovered from the polymerization reaction system.
Since the polycondensation reaction between a dihalo-aromatic compound and an alkali metal sulfide is a salt elimination reaction, a large amount of alkali metal halides (for example, NaCl) is formed as a by-product. In a recovery process, a reaction mixture containing the PAS produced and a large amount of byproducts is filtered, and then a resultant solid matter is washed to remove byproducts or oligomers mixed in or attached to the PAS. The thus recovered PAS is dried in a drying process. Further, there is also provided a process to recover an organic amide solvent, an unreacted monomer and the like.
In the initial stage of the development of PAS, there has been adopted a method of rapid cooling by removing a solvent by a solvent flashing process from a polymerization reaction system that is heated to high temperatures and in a high-pressure state, after the polymerization process. However, it is difficult to remove the byproducts and oligomers from the PAS by operations such as filtration and washing because the PAS is precipitated as fine powders, in the solvent flashing process. Further, the powdered PAS is difficult in handling and measurement.
Therefore, at present, there is adopted a method in which a slurry containing a granular PAS is formed by reducing the temperature of the reaction mixture containing an organic solvent, a PAS, a byproduct, a phase separation agent and the like, wherein the PAS is in a molten state, after the polymerization process. The phase separation agent has an effect to induce liquid—liquid phase separation in the reaction mixture in a high-temperature state, producing an organic amide solvent phase and a molten PAS phase. Organic carboxylic acid salts, water and the like are used as the phase separation agent. According to this method, a granular PAS can be recovered. The granular PAS is easy in separation of byproducts and oligomers and excellent in handling and measurement.
However, a method for reducing the temperature of a reaction mixture in a high-temperature state after a polymerization process requires a long cooling process, so the method has problems that the production efficiency is low and it is not economical. Therefore, several methods are proposed in order to produce a high-purity granular PAS in a relatively short cooling time.
Japanese Patent Laid-Open No. 2001-89569 proposes a method in which a polyhalogenated aromatic compound and a sulfidizing agent are subjected to polymerization reaction in a temperature range of 245 to 290° C., and then the reaction mixture is cooled in a two-step cooling rate in a cooling process. Specifically, it proposes a method in which the reaction mixture is first cooled at a cooling rate of 0.2 to 1.3° C./minute and, after reaching a specific temperature, at a cooling rate of higher than 1.3° C./minute. Its example shows an experimental example in which the reaction mixture was cooled at a cooling rate of 1° C./minute to 198° C. and then at a cooling rate of 2° C./minute to 50° C. The above patent describes that the method has provided a highly pure PAS in a short polymerization process time. However, this method cannot significantly reduce the cooling time.
Japanese Patent Laid-Open No. 10-87831 proposes a method for producing a granular PPS in which a sulfur source and a polyhalo-aromatic compound are subjected to polymerization reaction in an organic polar solvent in a sealed vessel and the reaction mixture is gradually cooled in the later stage of the polymerization reaction. According to the production method, the reaction mixture is gradually cooled in the later stage of the polymerization reaction, and the pressure of the vessel is relieved in a state where at least 50% by mole relative to the charged sulfur source of the polymer exists as a solid granular polymer and the pressure within the sealed vessel is 0.39×106 Pa or higher. Thus, a polymerization reaction mixture composed of a gas phase and a liquid phase is degassed and the pressure within the vessel is reduced. The above patent also describes that liquid—liquid phase separation is induced in a liquid phase component before cooling. This cooling method requires a gradual cooling, for example, at a cooling rate of about 1° C./minute, so it is difficult to substantially reduce the cooling time. Further, when the amount of water to be evaporated out of the gas phase by the degassing is large, this method requires a degassing time corresponding to the large amount of water.
Japanese Patent Laid-Open No. 09-296042 proposes a method for producing a PAS by reacting a dihalo-aromatic compound with a sulfidizing agent in an organic polar solvent. In the method, after the completion of the reaction, water is removed from a reaction slurry at a temperature lower than the temperature at the completion of the reaction and higher than the temperature where a polymer is precipitated, and then the reaction slurry is cooled to precipitate the polymer. The example in the above patent describes that after the completion of the reaction, the reaction mixture is cooled from 250° C. to 230° C. in 10 minutes; a water-NMP (N-methyl-2-pyrolidone) mixture is evaporated in 30 minutes while maintaining this temperature; and then the reaction mixture is cooled at a rate of 1° C./minute to 150° C. However, this method requires that water needs to be evaporated while maintaining the temperature where PPS is not precipitated, and also the reaction mixture needs to be gradually cooled at a cooling rate of 1° C./minute.
Japanese Patents No. 2604673 and No. 2604674 propose a method for producing a PAS by reacting an alkali metal sulfide with a dihalo-aromatic compound in an organic amide solvent, wherein a liquid phase is heated to a temperature exceeding the temperature of the liquid phase under the atmospheric pressure and a vapor phase part in a closed reactor is cooled, thereby condensing part of the vapor phase in the reactor and refluxing the condensate back into the liquid phase.
The method described in these patents is intended to return a large amount of reflux with a high water content back into the upper part of a liquid phase to form a layer with a high water content and to allow the layer to contain a residual alkali metal sulfide, a halogenated alkali metal, an oligomer and the like in the liquid phase. Therefore, this method is not a method for cooling a polymerization reaction system containing a liquid phase. These patents describe that during the cooling of the upper part of a reactor, a liquid temperature is maintained constant so that it is not reduced. Actually, cooling of a vapor phase only by this method cannot effectively cool the entire polymerization reaction system containing a liquid phase.